Java 5 (also known as Java 1.5) was a fairly major release that makes using Java a lot quicker , easier and cleaner (click here for a full list of the new features, including Meta-Data , concurrency, Generic Types, Profiling , integrated JMX Admin facilities and XML support as part of the java core.).
The problem is that a lot of Application Servers still require the previous (1.4) Version. Even if you’re running on a Server (such as JBoss) that does fully support Java 5, the chances are upgrading an existing application will take a lot of time , mainly in testing effort to make sure nothing has broken. As Java matures, running on older versions will become more and more of an issue.